Ring electrodes reaction with

The value of m is dependent upon the pH of the solution. If m 0, then redox reaction involves the incorporation and expulsion of protons during the redox reaction and this can be measured by a pH-sensitive bismuth oxide ring electrode [18,19]. In the special case when the ring electrode is a potentiometric sensor with a low exchange current density, such as the bismuth oxide sensor for pH [18,19], the ring electrode reaction does not significantly perturb the radial concentration profile of the disc product across the ring, and a different collection efficiency, termed the detection efficiency. 

Sakashita M, Lochel B, Strehblow HH (1982) An examination of the electrode reactions of Te, HgTe and Cdo.iHgo.sTe with rotating-split-ring-disc electrodes. J Electroanal Chem 140 75-89... 

Figure 5. Design of a cell for photoassisted electrolysis of C02 under elevated pressures.97 (1) Photoelectrode (2) reference electrode (3) counter electrode (4) sampling port with septum (5) pressure regulator (6) pressure gauge (7) O-rings (8) reaction cell (9) separator (10) quartz window (11) insulated connection (12) bolts (13) connections to potentiostat.

Fig. 11 Correlation between electrochemical potentials and OMTS bands for more than ten compounds including polyacenes, phthalo-cyanines, and porphyrins. OMTS data were acquired both from tunnel junctions and STM measurements. The standard potential relative to the normal hydrogen electrode associated with the half reaction M(solution) + e-(vac) —> M-(solution) is the y axis. The three outliers are assigned to the ring oxidation of porphyrins. (Reprinted with permission from [26])...

There are, however, obvious limitations. It is not possible to make a very small spherical electrode, because the leads that connect it to the circuit must be even much smaller lest they disturb the spherical geometry. Small disc or ring electrodes are more practicable, and have similar properties, but the mathematics becomes involved. Still, numerical and approximate explicit solutions for the current due to an electrochemical reaction at such electrodes have been obtained, and can be used for the evaluation of experimental data. In practice, ring electrodes with a radius of the order of 1 fxm can be fabricated, and rate constants of the order of a few cm s 1 be measured by recording currents in the steady state. The rate constants are obtained numerically by comparing the actual current with the diffusion-limited current. 

We consider the investigation of two consecutive electron-transfer reactions with a ring-disc electrode under stationary conditions. A species A reacts in two steps on the disk electrode first to an intermediate B which reacts further to the product C. The intermediate is transported to the ring, where the potential has been chosen such that it reants bank to A. The overall scheme is ... 

None of the set-ups discussed so far provides stirring of the electrolyte for bubble removal or for enhancement of the reaction rates. A standard set-up developed to study kinetic electrode processes is the rotating disc electrode [11]. The electrode is a small flat disc set in a vertical axle. The hydrodynamic flow pattern at the disc depends on rotation speed and can be calculated. An additional ring electrode set at a different potential provides information about reaction products such as, for example, hydrogen. However, because this set-up is designed to study kinetic processes and is usually equipped with a platinum disc, it becomes inconvenient if silicon samples of different geometries have to be mounted. 

The rotated ring-disc electrode (RRDE) has been shown to be an ideal tool for measuring the rate constants of very fast homogeneous reactions. In this method, we start with one reagent in the solution while the other is electrogenerated at the disc electrode, with the proportion of the latter that remains after reaction being monitored at the ring electrode. 

Frumkin and Nekrassow then applied Levich s equation to an analysis of intermediate production when the intermediate could leave the electrode surface, with the possibility of reacting again at the ring or leaving for the bulk. Damjanovic et aL developed the Ivanov and Levich equation to include a term, x, the ratio of the velocity of the two parallel reactions (A) and (B), thus increasing the helpful information obtained by using the equation. Damjanovic et al. s equation for the ratio of disk current to ring current is... 

Are Rotating Disk with Ring Electrodes Still Useful in the Twenty-first Century When the rotating disk electrode was first used, it was the 1960s since that time, many new methods for measuring electrode reactions (above all, spectroscopic ones, e.g., those in Section 7.5.15), have been invented. Furthermore, microelectrodes have made it possible, in effect, to reduce 5 by as much as 1000 times compared with that in a still solution, so that one of the uses of the rotating disk... 

Other Unusual Electrode Shapes. Various modifications of the rotating disk electrode are described in the literature. For example, the rotating disk electrode with ring sometimes suffers from bubbles that collect in the cotter and make it difficult to determine the fraction of the disk available for electrode reactions. One remedy is to use a cone-shaped electrode the ring is on the side of the cote and the disk at the tip. Bubbles don t like tips and skelter. 

This reorganisation also explains the decrease of the current of reduction peak IVC and the formation of the third oxidation peak (IVa). However, both peaks and also peak IIIC are too large to be explained by reduction or oxidation of adsorbed Co(II)TSPc only. It is assumed that this is the result of an electrocatalytic reaction, such as reaction with Co(II)TSPc in solution. This is confirmed by the fact that these peaks disappear when the Co(II)TSPc-modilied gold electrode is scanned in a pH 12 buffer in the absence of Co(II)TSPc in solution. In addition, the peak currents of peak IIIC in the first scan and peaks IIIC and IVC at the scan of maximum coverage vary linearly with Co(II)TSPc concentration (Fig. 7.5). Note that small peaks are observed at the same potentials where peaks IIIC and IVC occurred with solutions containing Co(II)TSPc. Electrochemical measurements of TSPc without Co show also a reduction wave at these potentials, explaining the ring reduction of CoTSPc in solution. This confirms the fact that Co(II)TSPc is adsorbed at the surface of the electrode and electro-catalyses the oxidation/reduction of Co(II)TSPc transported from solution towards the electrode surface. 

Y. Sekine and E.A.H. Hall, A lactulose sensor based on coupled enzyme reactions with a ring electrode fabricated from tetrathiafulvalen-tetra-cyanoquinodimetane, Biosens. Bioelectron., 13(9) (1998) 995-1005. 

With a concentric pair of electrodes the product from the disk electrode, which is produced at a given potential, is conveyed centrifically to the ring electrode (Figure 3.12). The latter usually is controlled at a different potential such that the product can be monitored. Because the relationship between the ring current and the disk current has been quantitatively established, the ring-disk electrode provides a means of measuring the kinetics of post-electron-transfer reactions of electrode products. 

Bipotentiostat — An instrument that can control the potential of two independent -> working electrodes. A - reference electrode and an -> auxiliary electrode are also needed therefore the cell is of the four-electrode type. Bipotentiostats are most often employed in electrochemical work with rotating ring-disk electrodes and scanning electrochemical microscopes. They are also needed for monitoring the electrode-reaction products with probe electrodes that are independently polarized. All major producers of electrochemical equipment offer this type of potentiostat. The instruments that can control the potential of more than two working electrodes are called multipotentiostats. 

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